s ⊆ s0 ++ a ⊦ a ∈ s0, a ∈ s, s ⊆ s0, s0 ++ a ⊆ s0
Used simplifier rules: eq: ⊦ a ∈ s → s ++ a = s;%sub-idem: ⊦ s ⊆ s; Used formulas: a ∈ s0